Is Today’s Climate Warmer than the Medieval and Roman Warm Periods?

In 2001, the U.N. Intergovernmental Panel on Climate Change (IPCC) featured a graph of Northern Hemisphere temperature history from a 1999 study by Profs. Michael Mann, Raymond Bradley, and Malcolm Hughes. Because of its shape, the graph became known as the “hockey stick.” From A.D. 1,000 to about 1915, the graph depicts a gradual decline in Northern Hemisphere temperatures (the hockey stick handle) followed by an abrupt upturn in hemispheric temperatures during the remainder of the 20th century (the blade).

The graph appears in the IPCC 2001 report’s Summary for Policymakers, Technical Summary, and chapter 2 on Observed Climate Variability and Change. Based on the Mann-Bradley-Hughes (MBH) study, the IPCC famously concluded that, “The 1990s are likely to have been the warmest decade of the millennium in the Northern Hemisphere and 1998 is likely to have been the warmest year” (chapter 2, p. 102). The IPCC also asserted that, “Evidence does not support the existence of globally synchronous periods of cooling or warming associated with the ‘Little Ice Age’ and ‘Medieval Warm Period’.” The hockey stick instantly became the poster child for pro-Kyoto advocacy, touted as seeing-is-believing evidence that late 20th century warmth was unprecedented during the past 1,000 years, and that mankind’s fuelish ways must be to blame.

Soon after its PR boost from the IPCC, the hockey stick became embroiled in a controversy that persists to this day. Books both pro and con have been written on the subject. Two leading critics, mining consultant Steve McIntyre and economist Ross McKitrick, argued that MBH’s computer program generates hockey stick-shaped graphs from random data. As for the IPCC’s dismissal of the Medieval Warm Period as a European phenomenon, the Center for the Study of Carbon Dioxide and Global Change maintains a large and growing archive of studies indicating that the Medieval Warm Period was global and/or warmer than recent decades.

A recent study published in Nature Climate Change further undermines the credibility of the hockey stick. The study, “Orbital forcing of tree-ring data,” by Jan Esper of Johannes Gutenberg University, in Germany, and colleagues from Germany, Switzerland, Finland, and Scotland, used X rays to measure changes in the cell-wall density of trees in Northern Finland over the past 2,000 years. The analysis examined both “living and subfossil pine (Pinus sylvestris) trees from 14 lakes and 3 lakeshore sites.”

The researchers argue that “X-ray densitometry” enables a more accurate reconstruction of climate history than does analyzing the width of tree rings — the principal data used by MBH. For example, MBH found a “divergence,” starting in 1960, between a decline in Northern Hemisphere temperatures, as reconstructed from tree ring data, and the increase in Northern Hemisphere temperatures, as measured by thermometers and other heat sensing instruments. The divergence raises the question of how MBH can be so sure the Medieval Warm Period was tiny or non-existent when their proxy data fail to reflect the instrument-measured warmth of recent decades. To give the hockey stick its dangerous-looking blade, MBH had to “hide the decline.”

In contrast, the Esper team found no divergence between instrumental data and temperatures inferred from density analysis of living trees in the study area.

So what’s the upshot? Their reconstruction “shows a succession of warm and cold episodes including peak warmth during Roman and Medieval times alternating with severe cool conditions centred in the fourth and fourteenth centuries.” The warmest 30-year period was A.D. 21-50, which was 1.05°C warmer than the mean temperature for 1951-1980 and ~0.5°C warmer than the region’s maximum 20th century warmth, which occured during 1921-1950.

The reconstruction also “reveals a long-term cooling trend of -0.31°C per 1,000 years (±0:03°C) over the 138 B.C.-A.D. 1900 period . . .” This trend is not reflected in tree ring width data from “the same temperature-sensitive trees.” Thus, reliance on such data (as in the hockey stick reconstruction) “probably causes an underestimation of historic temperatures.”

The authors write in a politic manner. Although they reference the MBH study, they do not directly criticize it or mention the hockey stick by name. They do not claim their reconstruction is definitive. However, they do argue that the reconstruction reflects long-term changes in “orbital configurations” that have continually reduced Northern Hemisphere summer “insolation” (solar irradiance) over the past two millennia. If so, then we should expect densitometry analysis of trees in other parts of the Northern Hemisphere to produce similar results.

Climate alarm skeptics will be pleased to see in the chart above evidence that the Roman Warm Period and Medieval Warm Period were warmer than the late 20th century. On the other hand, they may not be pleased by an apparent implication of the study. If Northern Hemisphere temperatures have been in an overall cooling trend for two millennia due to “orbital forcing” (i.e. reduced solar irradiance), then the burden of proof becomes greater on those who attribute the warmth of recent decades to solar variability rather than rising greenhouse gas concentrations.